Spoke a bit with some people in the Brocade stand at HP Discover in Barcelona. The open source / core http://www.vyatta.org/download will be kept, but could not get any commitment to what will happen to it or if they will update it. The 5400 (VSE, 6.6) and 5600 (VR, 7.x) are however available for a free 60-day trial. With the 5600 having a new architecture and is interfacing more closely with the hardware (using Intel's DPDK to for example dedicate processes to cores) that improves performance "quite a bit".
// Update, I since found out about VyOS: http://vyos.net/wiki/Main_Page which is a community fork of the Vyatta Core OS